code: def _norm(self, rng):
      if rng[0] == rng[1]:
          factor = 1 if rng[0] == 0 else 1e-6 * rng[0]
      else:
          factor = rng[1] - rng[0]
      return (self._scalars - rng[0]) / factor
  
file_path: mne-python/mne/viz/_3d_overlay.py
imports: from collections import OrderedDict
  import numpy
  from ..utils import logger
label_desc: Use image node_id 3 for calling the _Overlay obj's underlying member method code with example usage: obj._norm(rng) and returns: unknown